This invention relates to a low temperature heat pipe having means for removing contaminant hydrogen gas existing or collecting in the pipe which interferes with its heat transport capability. More specifically, this invention relates to an ammonia heat pipe employing a hydrogen getter material capable of removing hydrogen gas from the gaseous contents of the pipe. By the term "low temperature" as used herein is meant a temperature below 0.degree. C. (32.degree.F.) at which the heat pipe is operational.
A heat pipe is a heat transfer device having a high thermal conductance. It consists of a closed container made of a heat conductive material, usually tubular or cylindrical in shape, containing a heat transport medium. The container has at least one evaporation wall or section through which external heat is conducted to vaporize liquid heat transport medium and a condensation section or wall through which vaporized heat transport medium gives up its latent heat of vaporization to condense back to the liquid state. Condensed liquid returns to the evaporator section to complete a cycle which is repeated continuously as long as the temperature of the evaporator section is greater than the temperature of the condensation section and the heat transport medium is capable of evaporating and condensing at the temperatures of the sections. The net result is that heat is transferred from the environment surrounding the evaporation section. In most cases the condensation section of the heat pipe is located above the evaporation section in order that gravity assist the return of condensed vapor. Condensate may be returned from the condensation wall to the vaporization wall by way of a capillary structure such as metal gauze or capillary grooves located in the walls. Radiators such as metal fins are often mounted at the condensation section to assist heat transfer.
One of the largest uses of heat pipes at present is for permafrost stabilization for the trans-Alaskan pipeline. The trans-Alaskan pipeline stretches some 798 miles long. A little more than half of this pipeline, about 400 miles, is built above ground wherein the pipeline rests on a crossbeam attached to vertical metal support members imbedded in the ground. Much of the pipeline route is underlaid by permafrost which includes combinations of permanently frozen soil, ice and rock.
The heat pipes under these conditions are contained in the vertical support members and are designed to operate in colder months when the permafrost temperature at moderate depths (20 feet) is above the air temperature. Since the natural ground cover acts as a thermal control surface, any disturbance of this surface or any added heat flow into the ground along the metal support members could result in thawing of permafrost over a period of time. Such degradation of the permafrost could affect the vertical support members by reducing effective length, causing downdrag forces, lowering the point of fixity of the columns, decreasing the stability of slopes or increasing the potential for frost jacking which is the mechanism in which water, by alternately freezing and thawing, forces objects up and out of the ground.
Among the requirements for the heat transport media for heat pipes that are to be used at low temperature conditions are that it exist as liquid and a gas and have a vapor pressure of between about 5-200 psi at the desired operating temperature; low temperature conditions, such as exist along the Alaskan pipeline are in the temperature range of about -50.degree. F. to +100.degree. F., however even lower temperature conditions as may exist for example in outer space are applicable for the heat pipes described herein. Further, the heat transport medium must be safe to handle, be capable of being alternately vaporized and condensed under conditions of use; and that it be inexpensive and compatible with the material comprising the heat pipe. Typical heat transport media for use under low temperature conditions include halogenated hydrocarbons such as dichloridifluoromethane, dichlorotetrafluoroethane (Freons.RTM., E. I. Dupont DeNemours), 1,1-difluoroethane, 1,1,1,-chlorodifluoroethane, hexafluoroacetone, hydrocarbons such as butane and propane, ammonia, acetone, methyl chloride, ethyl chloride, methyl formate and ethyl amine. Preferred low temperature working fluids for use under arctic weather conditions, particularly during the winter months, are those having boiling points substantially below 0.degree. C., i.e. ammonia and low molecular weight halogenated hydrocarbons, i.e. Freons.TM., and particularly preferred being ammonia.
In addressing the problem of the degradation of the permafrost in the vicinity of the above ground Trans-Alaskan pipeline sections, about 120,000 carbon steel heat pipes using ammonia as the heat transport medium have been installed using two heat pipes for each vertical support member. During the winter months when the air temperature is below the ground temperature, the heat pipe functions to remove heat from the permafrost thus maintaining its integrity during the subsequent summer months when thawing can potentially occur.
A problem with the operation of the heat pipes is the presence of small amounts of non-condensable hydrogen gas which can collect, for example, by a corrosion reaction between water, which may be an impurity in the ammonia and the carbon steel of the pipe. The hydrogen gas accumulates primarily in the condenser section and inhibits the ammonia vapor from condensing at the top of the condensation section. This results in "condenser blockage" and leads to reduced heat removal capability. Thus, a means or method of removal of such contaminant hydrogen is vital if the permafrost is to be prevented from degrading.
In the art, the use of heat pipes in permafrost above-ground and underground structural assemblies, in which liquid ammonia can be a working fluid is well-known, as exemplified in the following patents: U.S. Pat. Nos. 4,036,286; 4,269,539; 4,194,856; 3,840,068; 3,788,389; 3,935,900 and 3,902,547.
Also, the use of a hydrogen getter in a heat pipe using water as the heat transport medium is exemplified in U.S. Pat. No. 4,043,387 which specifically discloses the use of tantalum, titanium or niobium as the hydrogen getter in the system. However, ammonia is not suggested or disclosed as an applicable heat transfer medium.
In addition, U.S. Pat. No. 4,159,737 describes a heat pipe having at least one getter being lanthanum, yttrium, or scandium, combined with barium, calcium or lithium, for removing gaseous impurities including hydrogen, and using liquid sodium, potassium or cesium, as the heat transport medium. The getter is described as extending from the vaporization wall to the condensation wall and is active at the operating temperature. However, the getter is described as not being very active at low temperatures.
Furthermore, metal alloys containing zirconium and manganese as components and mischmetal alloys which are useful as hydrogen storage materials are disclosed in U.S. Pat. No. 4,163,666; Japanese published Patent Applications Nos. 6125-201 and 6125-202; U.S. Pat. No. 4,228,145 and "Intermetallic Compounds" edited by J. H. Westbrook, Wiley (1967) New York, pp. 511-514 which discloses zirconium intermetallic compounds and their hydriding characteristics.
However, none of the above-cited references describe or specifically teach a heat pipe useful under low temperature conditions, such as arctic weather conditions, utilizing a low temperature working fluid, such as ammonia and further containing a hydrogen getter for successfully removing contaminant hydrogen gas from the system under the operating conditions. Further, what is not particularly taught is the gettering of contaminant hydrogen gas in the presence of oxygen-containing impurities, such as air and/or water, which may impede the gettering of hydrogen under the climatic conditions which the pipe is exposed to. Furthermore, it is not disclosed how to non-reversibly getter contaminant hydrogen gas under such conditions such as to effectively remove the gaseous hydrogen from the pipe heat exchanger system during its life of operation.